Add an Optional type to util

internal/util/util.go

@@ -91,17 +91,11 @@ func (this *Memo[T]) Value () T {
 // Invalidate marks the Memo's value as invalid, which will cause it to be
 // updated the next time Value is called.
 func (this *Memo[T]) Invalidate () {
+	var zero T
+	this.cache = zero
 	this.valid = false
 }
 
-// InvalidateTo invalidates the Memo and sets its value. The new value will be
-// entirely inaccessible. This is only intended to be used for setting a
-// reference to nil
-func (this *Memo[T]) InvalidateTo (value T) {
-	this.Invalidate()
-	this.cache = value
-}
-
 // Cycler stores a value and an accompanying io.Closer. When the value is set,
 // the closer associated with the previous value is closed.
 type Cycler[T any] struct {
@@ -130,3 +124,28 @@ func (this *Cycler[T]) Close () error {
 	this.closer = nil
 	return err
 }
+
+// Optional is an optional value.
+type Optional[T any] struct {
+	value T
+	exists bool
+}
+
+// Value returns the value and true if the value exists. If not, it returns the
+// last set value and false.
+func (this *Optional[T]) Value () (T, bool) {
+	return this.value, this.exists
+}
+
+// Set sets the value.
+func (this *Optional[T]) Set (value T) {
+	this.value = value
+	this.exists = true
+}
+
+// Unset unsets the value.
+func (this *Optional[T]) Unset () {
+	var zero T
+	this.value = zero
+	this.exists = false
+}